Thermal Physics (engineering) Flashcards
What is heat
energy flow due to a temperature difference. Energy always transfers from a higher temperature body, to a lower temperature body
What is internal energy
The sum of randomly distributed Kinetic and potential energies of all of its particles. Ideal gases has 0 potential energy
What is the first law of thermodynamics
Delta U = Q - W
(change in internal energy = head added - work done by the system)
What is the work done by expanding gas
Pressure * change in volume
What is a non-flow process?
Changes which occur in closed systems
What is adiabatic process?
a process where no heat leaves or enters the system
- if the gas expands, word is done by the system so temperature decreases
- if the gas is compressed work is done on the system so temperature decreases.
PV^the adiabatic constant = a constant
What is an isothermal change
Where the temperature remains constant so change in internal energies = 0
PV = constant
If the system is heated the work done by the system = energy transferred to the system
if the system is cooled the work dine on the system = energy transferred from the system.
What is the formula for work done at a constant pressure
W = P * delta V
What is the formula for a constant volume change?
Q = Delta U
What is a cyclic process
Where the system undergoes two or more processes, one after another, and returns to its initial V,T,P.
What is the work done on a P V diagram
Work done per cycle = area of loop.
What is an internal combustion engine>
An engine which contains cylinders of air which form systems. The air inside the cylinders is compressed when the engine is at a low temperature and expanded at a high temperature.
What are the engine cycles?pressure to increase
- Induction
- The piston moves down, causing the volume to increase, and so the volume of gas also increases. The gas mixture is drawn into the cylinder through an open inlet valve. - Compression
- The inlet valve is closed and the piston moves up, doing work on the gas, causing its volume to decrease and pressure to increase. The spark plug then ignites the gas mixture, causing the temperature and pressure to increase very rapidly - Expansion
- the gas mixture expands and so does work on the piston, causing it to move down the cylinder. As the gas is at a higher temperature, the work done by the gas here is higher than the work used to compress it. The exhaust valve opens and the pressure reduces to almost atmospheric. - Exhaust
- The piston moves up the cylinder, forcing the burnt gas out of the cylinder through the open exhaust valve. The pressure doesn’t return to atmospheric pressure
What is the Second law of thermodynamics
Heat engine convert heat energy into work. But heat is always lost to increasing the engines temperature. If the engine reaches the temperature of the heat source then no heat flows and no work is done. So Heat engines must operate between a heat source and a heat sink
Engine efficiency equation
Work done in Joules / heat transferred from the heat source in J
